Typical hydraulic systems utilize pilot stages to control large directional control valves. It is also well known to use electrical actuated pilot valves. For example, electrical actuated valves usually have two solenoids, one positioned on either side of the valve, to provide actuation of the spool in two directions. Additionally, the pilot valve may exhibit characteristics which achieve proportional performance, i.e. spool movement which is proportional to an applied current. However, the use of two solenoids per valve makes for a costly and a physically large system.
Casey et al. in U.S. Pat. No. 4,605,197 assigned to Fema Corporation discloses a pilot valve having only one linear motor. The linear force motor uses a permanent magnet. The permanent magnet allows the force motor to actuate the spool bi-directionally. However, the force motor design of Casey does present some problems.
For example, the force motor design of Casey operates as a three position device. For example, upon energization of the force motor the armature may travel to its limit. Alternatively, energization of the motor in the opposite direction causes the armature to travel to its other limit. The third or neutral position occurs when the linear motor is de-energized. In the de-energized condition a pair of springs counterbalance the magnetic force of the permanent magnet to center the armature between two pole pieces. However setting the initial flow position, i.e. positioning the spool lands to a predetermined position relative to the position of the control ports, may be difficult with only one neutral position. Moreover, much of the generated electromagnetic force is used to overcome the heavy force of the centering springs which results in inefficient operation of the force motor.
The present invention is directed to overcoming one or more of the problems as set forth above.